added duScrollOffset default value option for du-smooth-scroll

Hey! Great work on this, I love the smooth scrolling option especially. I found that adding a value to the module allowed me to set the offset once in my config section and made my HTML look a bit cleaner.
Thanks for taking this on - I was about to do the same thing, but luckily checked here first.
One comment about the implementation though. I think this will make it so that if you set a default offset, it cannot be changed via an attribute, which seems backwards.

You should just use the same technique as for duScrollDuration (line 16 below), like this...
```javascript
var offset = $attr.offset ? parseInt($attr.offset, 10) : duScrollOffset;
```